Radio frequencies form an electromagnetic spectrum which may be subdivided into spectrum bands. For a geographical area, regulatory authorities may define uses for different bands of spectrum. Certain sections of the spectrum may be reserved for air traffic communication, others for commercial radio channels, others for television broadcasts and yet others for cellular communication networks, for example.
As uses for spectrum have become varied, spectrum has become a scarce resource and it has become desirable to optimize use of spectrum. Allocating spectrum in a way that leaves a section dedicated for a certain use unused while other users suffer lack of spectrum is inefficient. For example, a spectrum band reserved for emergency communication may be used only intermittently while a spectrum band reserved for cellular communications may suffer multiple-user interference due to a high volume of data traffic in proportion to allocated spectrum.
Some communication devices are capable of using more than one spectrum band. For example certain cellular phones are configured to be capable of communicating using a first band and a first radio access technology, or alternatively a second band and a second radio access technology. Devices may select a less heavily loaded band to use. Multiband cellular devices are also capable of operating in different countries, where different spectrum bands are reserved for cellular use.
Cognitive radio is a technique wherein either a network or a wireless node changes its transmission or reception parameters, such as frequency band, to avoid interference caused from or to other users. Cognitive radios may be capable of sensing unused spectrum and taking advantage of it as long as it remains unused. Cognitive radios may be configured to detect that a user with higher access rights to the spectrum band becomes active, responsive to which the cognitive radio may leave the affected spectrum band to avoid causing interference to the user with higher access rights.